Sensitive equipment such as that found in IT data centres (i.e. computer servers and the like) usually includes critical elements for which it is important that there is no break in power supply during operation. Some critical structures may even be sensitive to ordinarily acceptable variations in power supplied from a utility (e.g. mains power).
Conventionally, power is supplied to such sensitive equipment using an uninterruptible power supply (UPS), which can guarantee a unbroken energy supply even during an outage in mains power (described herein as a grid outage). Typically an UPS can only cover a grid outage for a limited period of time. The time limit may arise if the UPS emergency power is sourced from a limited energy storage like re-chargeable batteries. The period of time where the UPS can bridge a grid outage is denoted the UPS runtime.
As many business come to rely on the permanent availability of their IT systems, so the need to provide an emergency power capability that can address a greater fraction of possible grid outages at a given location increases. In other words, it is desirable to increase the length of UPS runtime so that even long grid outages (the occurrence probability of which may be very low) can be handled without system downtime.
In addition to critical elements, which require an unbroken power supply, IT systems may include other loads (e.g. cooling systems or other support apparatus) which need to operate to ensure safe running of the critical elements. These less critical but nonetheless urgent elements (referred to herein as “essential elements”) may cope with a break in the power supply, but the duration of that break must be kept below a certain threshold. If the UPS runtime exceeds that threshold then it is necessary to include in the UPS functionality an ability to power the essential elements.
To address the demand for increased runtime, a typical UPS is fitted with either extended runtime battery storage or with a generator (often a diesel generator) that kicks in after a certain grid outage duration and thus limits the maximum grid outage duration that is seen by the UPS.
Increasing battery storage is simple but suffers from a number of drawbacks. Firstly, above a certain power level the physical amount of batteries required can grow beyond any practical level, whereby the solution becomes unpractical, unreliable and excessively expensive to maintain. Secondly, it can be difficult to power the essential elements because they are (by definition) not connected to the critical power line.
Providing a diesel generator is a more complex solution that suffers from other types of drawbacks. Firstly, diesel engines expose emission problems, noise and vibration. Secondly, the installation can be complex, which makes the cost per kW very high, especially for small systems/low power.
In practice the solution of increasing battery storage is often used for low power applications (typically below 10 kW) and a generator is used for important medium-high power applications (say 80 kW and up).
FIGS. 1 to 4 illustrate emergency power supply systems which embody the conventional principles discussed above. In FIG. 1, a UPS 100 is connected on a critical power line 102 between a data centre 106 and either mains power 108 or a diesel generator 110 depending on the status of an automatic transfer switch (ATS) 104. Essential loads are connected to the ATS 104 by a non-critical power line 112, which bypasses the UPS 100.
When a grid outage occurs, the UPS 100 is arranged to continue providing power to the critical loads using power from battery 114 as an input. The ATS 104 is arranged to switch from the mains power 108 to the generator 110 after a certain grid outage duration. When the supply from the generator 110 is online the essential loads will begin to receive power again and the UPS 100 can source power from the generator 110 from its inlet to power the critical loads and recharge the battery 114.
The essential loads suffer a downtime until the generator 110 kicks in. The size of the battery 114 is typically chosen to cope with most start-up obstacles of the generator 110 such that the power sources for the UPS 100 do not fail even if several start attempts or even minor repairs are needed to the generator.
The system shown in FIG. 2 is similar to that shown in FIG. 1, and where appropriate the same reference number is used for like components. In FIG. 2 the generator is implemented an AC fuel cell generator comprising a fuel cell 116 connected via a DC/AC converter 118 to the ATS 114. This arrangement may provide a faster and more reliable generator start-up, so the battery 114 may be smaller than the FIG. 1 system.
The system shown in FIG. 3 is similar to that shown in FIG. 2, and where appropriate the same reference number is used for like components. In this system the UPS 100 will never see a shortage of battery power (i.e. it is an “infinite battery” arrangement) because the fuel cell 116 is connected via a DC/DC converter 120 to the UPS battery input in parallel with the battery. However, in this system the essential loads are not supported.
The system of FIG. 4 is a special variant of the FIG. 3 arrangement which is disclosed fully in US 2008/0067872. In this arrangement the UPS 100 contains a bi-directional converter which allows the UPS 100 to supply the essential loads during a grid outage by the fuel cell generator.
FIG. 5 shows a typical implementation of double conversion UPS which is suitable for use in the systems shown in FIGS. 1 to 3. The UPS 100 consists of a rectifier 122 that converts power delivered from the ATS 104 (e.g. from the mains 108 or a generator during mains outage) into DC for supplying a DC bus 126. The DC bus 126 is connected to the critical load line 102 via an inverter 124 that converts power on the DC bus into AC power having the correct voltage and a frequency that is independent of what goes on in the grid supply.
The UPS may be distributed over a number of racks within the data centre. Each rack may have a UPS module 130 associated with it, each UPS module 130 containing a rectifier 122 and inverter 124. The mains power (power from ATS 114) is distributed to the inlets of the UPS modules via a grid AC rail 128 and outputs of the UPS modules are all fed into an outlet AC rail 132 that feeds the critical power line 102.
In a modular system like this one can individually dimension the system to match the demand for power capacity and add extra UPS modules beyond the needed net power consumption in order to offer redundancy and thus increase of power availability through fault tolerance.
For example, in a system where the net power demand is 30 kW, the net power may be covered by three 10 kW UPS modules, but an additional module may be added to provide N+1 redundancy (the modules share the load) thereby offering the ability to lose one module due to a fault without failing to deliver full power to the critical load.